Low k dielectric surface damage control

ABSTRACT

A method of removing a silicon nitride or a nitride-based bottom etch stop layer in a copper damascene structure by etching the bottom etch stop layer is disclosed, with the method using a high density, high radical concentration plasma containing fluorine and oxygen to minimize back sputtering of copper underlying the bottom etch stop layer and surface roughening of the low-k interlayer dielectric caused by the plasma.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/457,888filed Jul. 17, 2006, which is a continuation-in-part of application Ser.No. 10/701,825, filed Nov. 5, 2003, abandoned.

FIELD OF THE INVENTION

The present invention relates to a method of forming a copper damascenestructure in a semiconductor device, and more particularly to a methodof removing a silicon nitride or a nitride-based bottom etch stop layerin a copper damascene structure having a porous low K interlayerdielectric.

BACKGROUND OF THE INVENTION

In the conventional process for forming copper damascene structures insemiconductor devices, after the damascene opening has been etched intothe porous low-k interlayer dielectric (ILD), the bottom etch stop layeris etched with a dry etch process before the damascene opening is filledwith copper metal. A number of materials may be used for the bottom etchstop layer. Silicon carbide and silicon nitride are examples ofmaterials commonly used for this purpose. Where the bottom etch stoplayer is silicon nitride, the dry etch process conventionally practicedis plasma etch with a bias power. However, this etch process isgenerally conducted with a very low bias power because any “overetch” ofthe silicon nitride layer will cause undesirable back sputtering of theunderlying copper in to the via. Such back sputtering of the underlyingcopper is not desirable because the sputtered extraneous copper depositson the sidewalls of the low-k ILD can cause reliability problems.

Thus, an improved method of etching the silicon nitride bottom etch stoplayer in a copper damascene structure is desired. The concerns discussedherein are equally applicable to single damascene structures, copper viastep structures, and copper dual damascene structures (with or withoutan intermediate etch stop layer).

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, disclosed herein isa method of removing a nitride-based bottom etch stop layer in a copperdamascene structure, by etching the bottom etch stop layer using a highdensity, high radical concentration plasma containing fluorine andoxygen. The copper damascene structure may be a via step, a singledamascene structure, a dual damascene structure, or a non-intermediateetch stop layer dual damascene structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional illustration of a non-intermediate etch stop layerdual damascene structure at an interim process step before the removalof the bottom etch stop layer; and

FIG. 2 is a sectional illustration of the non-intermediate etch stoplayer dual damascene structure of FIG. 1, after the bottom etch stoplayer has been removed.

DETAILED DESCRIPTION OF THE INVENTION

According to an embodiment of the present invention, disclosed herein isan etch process for removing a nitride-based bottom etch stop layer in acopper damascene structure. The method according to the presentinvention is applicable to a variety of copper damascene structures,such as, for example, a single damascene, a dual damascene, anon-intermediate etch stop layer dual damascene, and a via stepstructure.

FIG. 1 illustrates a typical non-intermediate etch stop layer dualdamascene structure at an interim stage of processing where a trench 10and a via 20 openings have been formed in low-k interlayer dielectric(ILD) 30 but bottom etch stop layer 40 is still intact. Various othermaterials may be used for bottom etch stop layers, but the method of thepresent invention is applicable to those copper damascene structuresutilizing a nitride-based bottom etch stop layer. The bottom etch stoplayer 40 may be composed of silicon nitride or other nitride-basedmaterials such as oxynitride, a mixture of silicon oxide and silicon.

According to an embodiment of the present invention, the bottom etchstop layer 40 at the bottom of the via 20 is etched using a highdensity, high radical concentration plasma containing fluorine andoxygen. The high radical concentration in the plasma is defined ashaving a radical-to-ion ratio equal to or greater than about 10:1. By“radical-to-ion-ratio,” it is meant the ratio of (1) the combination offluorine and oxygen radicals to (2) the combination of fluorine andoxygen ions.

Thus, a method of etching a nitride-based bottom etch stop layer in acopper damascene structure is also disclosed, with the methodcomprising: etching the nitride-based bottom etch stop layer using ahigh density, high radical concentration plasma containing both fluorineand oxygen, wherein the fluorine and oxygen radical to fluorine andoxygen ion ratio in the plasma is equal to or greater than about 10:1.

By keeping the amount of ion in the plasma low, back sputtering of thecopper underneath the bottom etch stop layer is minimized, while alsominimizing the physical damage to the surface of the low-k ILD by theplasma. If the radical-to-ion ratio is less than about 10:1, there is agreater likelihood that the underlying copper 50 will back sputter anddeposit on the low-k ILD sidewalls of the damascene via 20, which maycause reliability issues. Also, plasma containing a higher ion contenthas a tendency to cause physical damage on the exposed horizontalsurface 35 of the low-k ILD in the dual damascene structure during theetch process, producing a rough low-k ILD surface. The rough surface isnot desirable because it will increase the copper layer's sheetresistance, Rs, in the final copper damascene structure, especially innarrow lines.

In a preferred embodiment of the present invention, high density plasmamay be produced utilizing one of a variety of available methods, suchas, for example, inductive coupling plasma, electron cyclotronresonance, helicon wave, surface wave, and some capacitive couplingplasma, and microwave plasma tool. The use of a high density plasmasource is to have high dissociation so as to create more free fluorineor oxygen radical. A high radical concentration is helpful forcontrolling the bottom etch stop layer's edge profile around the etchedarea. When the bottom etch stop layer 40 is removed from the bottom ofthe via 20 using the high radical-to-ion ratio plasma containingfluorine and oxygen, the edge profile of the bottom etch stop layeraround the opening is vertical rather than tapered. The fluorine in theplasma may be provided by at least one of CF₄, CHF₃, SF₆, NF₃, C₂F₆,C₄F₈, CH₂F₂, CH₃F, and C₄F₆. High radical concentration also increasesthe etching process throughput.

By using a high density, high radical-to-ion ratio plasma containingfluorine and oxygen, a more chemical and less physical plasma etching isachieved, thus effectively removing the nitride-based bottom etch stoplayer, while minimizing the back sputter of the underlying copper andthe surface damage of the low-k ILD in the damascene opening formed bythe trench 10 and via 20. FIG. 2 illustrates the non-intermediate etchstop layer dual damascene structure of FIG. 1 just after the bottom etchstop layer 40 has been etched away, using the process according to anembodiment of the present invention from the bottom of the via 20.

While the foregoing invention has been described with reference to theabove embodiments, various modifications and changes can be made withoutdeparting from the spirit of the invention. Accordingly, all suchmodifications and changes are considered to be within the scope of theappended claims.

1. A method of etching a nitride-based bottom etch stop layer in acopper damascene structure comprising: etching the nitride-based bottometch stop layer using a high density, high radical concentration plasmacontaining both fluorine and oxygen, wherein the fluorine and oxygenradical to fluorine and oxygen ion ratio in the plasma is equal to orgreater than about 10:1.
 2. A method according to claim 1, wherein thenitride-based bottom etch stop layer is silicon nitride.
 3. A methodaccording to claim 1, wherein the nitride-based bottom etch stop layeris oxynitride.
 4. A method according to claim 1, wherein the fluorine isprovided by at least one of CF₄, CHF₃, SF₆, NF₃, C₂F₆, C₄F₈, CH₂F₂,CH₃F, and C₄F₆.
 5. A method according to claim 1, wherein the highdensity plasma further comprises N₂ and any one of inert gases.
 6. Amethod according to claim 1, wherein the copper damascene structure is avia step.
 7. A method according to claim 1, wherein the copper damascenestructure is a single damascene structure.
 8. A method according toclaim 1, wherein the copper damascene structure is a non-intermediateetch stop layer dual damascene.